Braking forces disengage the rear ride height device automagically. Same mechanism used at the start and during the race but different methods of activating it.
From motomatters:
The device functions as follows: before the start of the race (or before a practice start), the rider uses their weight to compress the suspension, then twists the lever to engage the mechanism. The mechanism then locks the rear suspension in a lower position for the duration of the start. When the rider arrives at the first corner, and brakes hard before entering the turn. That hard braking causes the rear to lift, which in turn triggers the unlocking mechanism, which frees the device and returns the rear suspension to its normal operation.
I believe the button is for opening the hydraulic valve to get it to squat not for disengaging it
